The RF photonic link of this disclosure has enhanced linearity because 3rd order distortion products are suppressed. This suppression of the 3rd order distortion is useful for RF photonic links that carry RF signals having a signal bandwidth that is no larger than one octave. Grating modulators generally are considered to have poor linearity. The presently disclosed technology achieves high-linearity performance from grating modulators.
There are a number of commercial applications of RF photonic links. These applications include fiber radio in which signals for wireless RF networks (such as cell phone networks) or links are transported to/from the base stations through optical fiber. These signals have limited bandwidth but are at high carrier frequency, with the carrier frequency ranging from 1 to 60 GHz. The bandwidth of these signals is generally less than 5-10% of the carrier frequency. The signal bandwidth for defense applications can be even larger but many applications still have a bandwidth of less than one octave. The enhanced SFDR (Spur Free Dynamic Range) of the presently disclosed technology is well suited to both the commercial applications and the defense applications.
Some prior optoelectronic modulators have achieved enhanced linearity in their modulation performance by operating them at conditions for which the 3rd derivative of the modulator transfer function is minimized. An article by Farwell et al. (IEEE Photonics Technology Letters, vol. 5, no. 7, July 1993, pp. 779-782) describes applying a DC bias voltage, in addition to the applied RF signal, to a Mach-Zehnder interferometric modulator so that its operation about that bias point has reduced 3rd order intermodulation products. Other articles, by Welstand et al. (IEEE Photonics Technology Letters, vol. 7, no. 7, July 1995, pp. 751-753), and by Sun et al. (Electronics Letters, vol. 31, no. 11, 25 May 1995, pp. 902-903) describe applying a DC bias to an electro-absorption modulator such that the transfer function of that modulator has a null in its third derivative for a specific laser wavelength. The presently disclosed technology does not need to apply a DC bias voltage but rather can use the choice of a particular laser wavelength to determine the operating point of the modulator. The primary difference between the present disclosure and these prior art references is the use of the grating modulator.
A prior enhanced-linearity link is described in an article by Johnson and Roussell (IEEE Photonics Technology Letters, vol. 2, no. 11, November 1990, pp. 810-811). This link supplies laser light at two optical polarizations, both TE and TM, into the optical waveguide of the modulator. For light of TM polarization, the transfer function of a Mach-Zehnder interferometric modulator has a positive slope with a first shape; for light of TE polarization, the transfer function of that modulator has a negative slope with a second shape. This prior link achieves a suppression of the 3rd order intermodulation distortion in the modulated output by supplying input light of a specific ratio of TE and TM polarizations into the modulator and by applying a specific DC bias voltage to the modulator. At this bias voltage, the 3rd derivative of the transfer function for TE polarization has the opposite sign as the 3rd derivative of the transfer function for TM polarization. Thus, the 3rd-derivative contributions from a combination of both the TE and the TM polarized modulated light can cancel each other. The combination of the TE and TM polarized modulated light actually reduces the effective amount of the linear modulation (arising from the first derivative or slope of those transfer functions, since those slopes also have opposite signs and thus their contributions subtract from each other, too). However, that relatively small reduction in modulation efficiency is acceptable given the substantial suppression of the intermodulation distortion. In contrast to this prior art, for the present disclosure, the contributions to the linear modulation (first derivative of the transfer function) from the two laser wavelengths add to each other whereas the contributions to the distortion (third derivative of the transfer function) cancel each other.
Optical modulators based on a grating formed in an optical waveguide have been described in articles by An, Cho and Matsuo (IEEE Journal of Quantum Electronics, vol. QE-13, no. 4, April 1977, pp. 206-208), by Cutolo et al. (Applied Physics Letters, vol. 71, no. 2, 14 Jul. 1997, pp. 199-201) and by Kim et al. (Electronics Letters, vol. 41, no. 18, 1 Sep. 2005). FIG. 3 shows an illustration of such a grating modulator. This modulator has a waveguide with a grating form in it, similar to the waveguide shown in FIG. 2a. A pair of electrodes is formed alongside the grating waveguide and the RF modulating signal is applied to these electrodes, producing an electric field across the grating waveguide. In general, these prior modulators use non-travelling-wave (or “bulk”) RF electrodes to apply the modulation controlling electric field. Those RF electrodes are not part of any RF waveguide. Those “bulk” electrodes typically are connected to an RF signal source by means of an RF cable and wires and represent the termination point for the RF cable. Although FIG. 3 shows laterally placed electrodes, one of the electrodes also could be placed above the grating waveguide, with the other electrode (or electrodes) placed at the sides of the waveguide or at the bottom of the substrate containing the waveguide.